The present invention relates to nanoparticles and microparticles and, more particularly, to a method for producing solid particles with sizes in the nm-μm range.
Nanoparticles and microparticles are becoming increasingly important in various technological applications, inter alia, paints and inks, powder metallurgy, drug delivery, and dry lubricants. Paints and inks are often composed of particles, suspended in a liquid carrier. After being applied to a surface, the liquid carrier evaporates, leaving a solid residue composed of the conglomerated particles. In some technological applications, it is advantageous to have particles with particular electrical or magnetic properties. For example, particles with a high magnetic permeability may be useful in paints that absorb electromagnetic radiation, e.g., radar waves. Particles having an electrical charge may be useful in printing processes in which the direction of the ink jet is controlled with electrostatic steering.
Powder particles are also used in the fabrication of metal and ceramic components, by pressing and sintering. For example, particles such as tungsten carbide (WC) are sintered to form hard cutting tools. Sintered components generally have some degree of porosity, due to the voids between particles. These voids may adversely affect their characteristics. It is believed that the porosity can be reduced, and the properties improved, by using a mixture of powders that includes very small sizes, often termed “nanoparticles”, which can fill the voids between larger particles.
Nanoparticles have also been proposed as a drug delivery means. In some such applications, it may be advantageous to have nanoparticles having a nanostructured carbon surface, to which therapeutic molecules can be bound.
Hollow-layered, spherical MoS2 and WS2 nanoparticles have been shown to have excellent lubricating properties, perhaps by acting as ball-bearings on a nanometric scale (Rapoport L, Bilik Y, Feldman Y, Hamyonfer M, Cohen S. R, Tenne R, “Hollow Nanoparticles of WS2 as Potential Solid-State Lubricants”, Nature, 387 (6635), 791-793, 1997; Rapoport L, Fleisher N, Tenne R, “Fullerene-like WS2 Nanoparticles: Superior Lubricants for Hard Conditions”, Adv. Mater., 15 (7-8), 651-655, 2003). These hollow-layered, spherical nanoparticles are currently manufactured using a lengthy, high-temperature, gas-phase process.
Nanoparticles and microparticles having an electrical charge or a high magnetic permeability may be particularly useful in the rapidly developing field of micro-electro-mechanical systems (MEMS), and particularly in the sub-field wherein fluid elements are integrated with electronic elements on a single chip. Electric or magnetic fields can readily be coupled to charged or magnetic particles respectively, and thus in some applications, it may be advantageous to add particles to the fluid as a means of exerting force on the fluid. Thus, forces coupled to the particles could be transferred to the adjacent fluid, and used to propel the fluid along a channel, or to encourage mixing between two fluids in which the particles are suspended.
Nanoparticles and microparticles are currently manufactured by a variety of chemical and physical processes. In one process, various carbon particles, including carbon nano-tubes and carbon nano-onions are produced by maintaining a d.c. arc discharge between graphite electrodes submerged in a passive liquid environment, such as liquid nitrogen or water (see Sano N., Wang H., Chhowalla M., Alexandou I, Amaratuunga G. A. J., Nature, 414, 506, (2001); Sano N., Wang H., Chhowalla M., Alexandou I., Teo K. B. K., Amaratuunga G. A. J., J. Appl. Phys, 92, 2783, (2002)). The carbon nanoparticle is constituted from material eroded from the graphite electrodes. The liquid is believed to play relatively passive roles: evaporation from the liquid provides a vapor bubble atmosphere in which the arc operates, and the liquid may also cool the vapor and quench metastable structures.
Nanoparticles, and in particular, carbon nanotubes, have also been produced using a pulsed arc in air, as taught in our co-pending U.S. patent application Ser. No. 10/615,141 (filed Jul. 9, 2003 and published as United States Patent Application No. 20040026232), which is not to be construed as prior art with respect to the instant application. In this technique as well, the material constituting the particles is eroded from the electrodes, e.g., graphite electrodes in the case of carbon nanotubes. However, unlike the art taught by Sano, et al., a short, pulsed arc is employed, and the fluid contributes material for incorporation into the nanoparticles.
There is therefore a recognized need for, and it would be highly advantageous to have, a method for producing nanoparticles and microparticles that is simple, efficient, and inexpensive with respect to known methods. It would be of particular advantage to have a method for producing nanoparticles and microparticles that incorporates mass from the liquid of the surrounding medium into these nanoparticles and microparticles.